1. Field of the Invention
The invention relates to the field of printing inks for paper for graphic purposes consisting of pigment, an alkyd resin modified with long-chain fatty acids and, optionally, conventional additives, such as, lubricants, solvents, and viscosity modifiers. Moreover, the invention relates to the use of these printing inks.
2. Description of the Prior Art
Modern printing inks for printing paper must meet a large number of requirements which are dependent on the particular final application and often appear to be contradictory.
For example, paper printing inks for holding boxes should have an abrasion resistance as high as possible. On the other hand, it should be possible to readily remove these printing inks, for example, from newspapers, bulk printing matter, etc., so that these products can be processed as used paper and recycled.
A further requirement is that printing inks used in the manufacture of packaging materials for foodstuffs, luxury foods and cosmetics, such as, coffee, alcohol, tobacco, or chocolate wrappers, must have an extremely neutral odor and taste. In the area of printed labels glued onto reusable bottles, it is necessary that the printed labels have a high rate of penetration for suds, so that cleaning liquors are able to rapidly penetrate the two-dimensional film of printing ink on the label and remove it easily.
From a production point of view also, printing inks and their components must fulfill a number of requirements. For instance, it is necessary to ensure good wetting of the pigments by choosing appropriate binder compositions, by special additives, or by pretreating the pigments. However, the pigment wettability should not be achieved at the expense of the printing quality. Moreover, the composition of a printing ink should be such, that it has as high a pigment absorption capacity as possible, without disadvantageously impairing the adhesion of the printing.
These complex application and manufacturing requirements are the cause for the numerous attempts to find an acceptable compromise, see, for example, British Pat. No. 699,513, U.S. Pat. Nos. 3,481,891 and 3,786,008.
Besides pigments, the usual printing inks for paper contain binders and additives, which aid the processability. The binders regularly comprise a resin mixture of drying alkyd resins, for example, vegetable oils, such as, linseed oil, soybean oil, tung oil, including dryers, such as, cobalt naphthenate and manganese octoate, as well as hard resins, such as, phenol-modified rosin derivatives, hydrocarbon resins and others. Moreover, the usual printing inks contain considerable amounts of mineral oils in the range of 25 to 40 weight percent.
The oxidatively cross-linking alkyds have the particular task of aiding the wetting of the pigments. Usually for this purpose, phthalate resins (ortho-, iso-, and terephthalate esters) and/or urethane-containing alkyd resins are used. They are composed of dicarboxylic acids and multifunctional alcohols, such as, glycerine, pentaerythritol, sorbitol, trimethylolpropane, or of multifunctional isocyanates on the one hand and oils, natural or synthetic fatty acids or fatty alcohols on the other. These materials are produced by conventional methods. Because of their better pigment absorption capability, alkyds of low viscosity are generally preferred insofar as their use is permitted by the consistency of the finished printing ink.
For the preparation of the printing ink, various resins are dissolved at elevated temperatures in mineral oil and the pigment is then added. The printing ink film, produced with such conventional printing inks, then cures oxidatively.
The properties of such printing inks and the printings produced with them have not been entirely satisfactory. Improvements in the printability properties are needed. Moreover, in previously known printing inks, contact yellowing and odor formation interfere with the printed products. The increasing desire to recycle used paper has now posed an additional special problem resulting from the use of these prior art printing inks. This can be observed by comparing the degree of whiteness of printed and unprinted paper, and especially of imitation art paper.